<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <ul>
        <li>123</li>
    </ul>
    <script>
        //1.创建节点元素节点
        var li=document.createElement('li');
        //2.添加节点node.appendChild(child) node父级 child子级
        var ul=document.querySelector('ul');
        //后面最加元素,加到末尾
        ul.appendChild(li);
        //3.添加节点 node.insertBefore(child,指定元素);
        var lili=document.createElement('li');
        ul.insertBefore(lili,ul.children[0]);
        //4.我们想要页面添加一个新元素:1.创建元素2.添加元素
    </script>
</body>
</html>